A factor in explaining Presidential election outcomes is that the presidential election winner has been from the same party 3 times in a row only once after FDR — that was with H W Bush in 1988. Of the 8 times the same party won the presidential election the previous two times, only once did the party in power win a third time. If winning presidential elections was a 50-50% coin toss, on average this should happen four times.
If HRC won, that would have been the second time.
Republicans also do well with candidates over the age of 60 while Democrats don’t. Trump, H W Bush, Reagan and Eisenhower were all over 60 when first elected — W Bush and Nixon were under 60. The last Democrat to win who was over 60 is Buchanan in 1856.
Whether the candidate was HRC or Bernie, Democrats would still have been running against these two currents of history.
The above is not to say winning a third term is not possible, but it should be recognized as being a more difficult situation and one should campaign as a third term if something special can be taken advantage of.
Democrats also have very old leadership. In the House, the Democratic minority leader and whip are 76 and 77. Senate Minority Leader Schumer is 66. This is not to say they are not good, loyal or effective. The problem is that younger candidates that can best win are not given the chance to grow.